1. Field of the Invention
The present invention relates to an optical filter and image pickup apparatus which uses an aperture device having the optical filter.
2. Description of the Related Art
An optical device such as a conventional video camera is provided with a light quantity aperture device to control an incident light quantity to a solid-state image sensor. The light quantity aperture device is controlled to decrease its aperture size when an object is bright. An ND (Neutral Density) filter is arranged in the light quantity aperture device to prevent the aperture from becoming extremely small even when the object is bright, thus preventing a hunting phenomenon or light diffraction phenomenon.
As examples of the ND filters, other than those having a single density, those having a transparent portion and a portion with a transmittance that changes continuously or stepwise are known, as shown in, e.g., Japanese Patent Laid-Open No. 6-265971 and 2004-205951.
The sensitivity and resolution of a solid-state image sensor in an optical device such as a video camera have been improving. Accordingly, if an ND filter arranged in the light quantity aperture device has a high spectral reflectance in the entire visible light wavelength range, ghost and flare tend to occur on the image pickup screen. To decrease the ghost and flare, it is important to decrease the spectral reflectance of the ND filter.
To decrease the spectral reflectance of the ND filter, a method of forming an anti-reflection film on an ND film by vacuum deposition or the like is known. Also, a method of forming an AR (Anti-Reflection) coat as an anti-reflection film on a surface where the ND film is not formed is also known. When forming an anti-reflection film comprising a single layer film on a substrate surface, however, although the reflectance can be decreased for a specific wavelength, it cannot be decreased for the other wavelengths. To cope with this situation, as shown in Japanese Patent Laid-Open Nos. 8-075902, 10-133253, and 2003-344612, suppression of the reflectance for an arbitrary wavelength range by forming several types of thin films having different refractive indices, e.g., SiO2, MgF2, Nb2O5, TiO2, Ta2O5, ZrO2, and Al2O3, is known.
When employing the ND filter as described above in the aperture device which is used in an optical device such as a video camera, the following problems remain.
More specifically, when a beam passing through an image pickup optical system including the aperture device of the camera forms an image on the surface of a solid-state image sensor, the surface of the solid-state image sensor or of a lens between the aperture device and image sensor may reflect part of the beam to return it to the aperture device. If the ND filter reflects the reflected light again so the light enters the solid-state image sensor again, ghost or flare occurs.
To prevent this, generally, the ND filter is arranged in the aperture device such that the ND film surface coated with an anti-reflection film is on the side of the solid-state image sensor. This, however, cannot prevent the reflected light from being transmitted through the ND film of the ND filter and through the substrate of the filter, and reflected by the boundary surface on the opposite side to travel toward the solid-state image sensor again. When ND films are formed on the two sides of the substrate, the reflected light cannot be prevented from being reflected by the boundary surface of the substrate and the ND film which is formed on the opposite side of the solid-state image sensor.
Moreover, the ND filter may have a transparent region on it. If a transparent anti-reflection multilayer film is used to prevent reflection by the transparent region, the material having an optimal refractive index as the multilayer film is limited, making it sometimes difficult to achieve an optimal combination. Also, in addition to the ND film, the anti-reflection film must be formed as a multilayer film, leading to an increase in number of manufacturing steps.